An AC/DC power converter is used to drive a string of LEDs. The AC/DC power converter includes a rectifier circuit for rectifying an AC input voltage into a DC voltage. The DC voltage provides input current to the LEDs. An isolated AC/DC power converter includes a transformer that isolates the output (secondary side) from the input (primary side) of the converter and therefore separate grounds are used for the input and the output of the isolated converter circuit. The AC/DC power converters may include PFC circuitry in the primary side that controls the input current so that the input current stays closely in phase with the waveform of the AC input voltage (e.g., sine wave).
Power converters generally incorporate two modes of operation when dealing with heavy as well as light loads. In Critical Conducting Mode (CrM) the switching converter initiates a new switching cycle immediately after the inductor current in the switching converter goes to zero. In Discontinuous Conduction Mode (DCM) the switching converter initiates a new switching cycle much after the inductor current goes to zero and is typically used for light loads. The CrM is preferred over the DCM because a smaller switching transistor and a smaller transformer are used for CrM. The CrM is commonly selected for full or heavy loads but the load range for the CrM is limited. For light loads the DCM is preferred.